It is well known that vacuum gas oils resulting from thermal conversion processes, such as visbreaking and coking, are particularly rich in basic nitrogen compounds. These vacuum gas oils, particularly waxy hydrocarbons, are submitted to catalytic cracking in order to form lighter hydrocarbons. Moreover, it is well known that the catalysts used in catalytic cracking, particularly zeolites, are poisoned by basic nitrogen compounds which are contained in gas oils. The poisoning of the acid sites of the catalysts leads to a reduction of the conversion rate of as much as 10 to 20%.
The nitrogen compounds which are contained in hydrocarbon feedstocks, typically the vacuum gas oils, are present for a major part in the form of basic compounds. In order to render the gas oils suitable for further treatment, it is necessary to reduce considerably the amount of the basic nitrogen compounds.
It has already been proposed to treat hydrocarbon feedstocks having a boiling point between 200.degree. and 650.degree. C., which have to be further submitted to thermal or catalytic cracking to form gasolines, with an organic or inorganic acid, but this treatment only partially eliminates the nitrogen compounds. Moreover, when a treatment with concentrated sulphuric acid is used, part of the compounds of the feedstocks is transformed by sulfonation, causing significant loss.
The present assignee's Belgian Pat. No. 884149 discloses treating liquid hydrocarbon mixtures with very dilute aqueous acid solutions. This technique may be applied to the gas oils, but with difficulties due to their high content in basic nitrogen compounds and due to the low efficiency of the diluted solutions on the vacuum gas oil feed.
There exist still other processes which use a packed column and medium concentrated acid solutions in the order of 65%. These processes concern particularly liquid hydrocarbons and therefore could not be easily adapted to vacuum gas oils, which are present in the form of a relatively viscous wax.
Therefore there is a need for an improved process to remove the nitrogen impurities from vacuum gas oils.